1. Field of the Invention
The present invention relates to an optical fiber end processing method and an optical fiber processing apparatus, and further specifically relates to the optical fiber end processing method and the optical fiber end processing apparatus for fabricating an end portion of an optical fiber, with a mode field diameter (core diameter) expanded by thermally diffusing a dopant included in a core, etc., of an optical fiber.
2. Description of the Related Art
When single mode fibers with different mode field diameters are required to be connected or spliced, reduction of connection loss or splice loss is achieved by a technique of diffusing a dopant by applying heat treatment to an optical fiber end portion with smaller core diameter, and connecting (or splicing) the single mode fibers to make the mode field diameter comparable with that of another fiber. This technique is called TEC (Thermally diffused Expanded Core), and is effective not only for the connection of the optical fibers, but also for a case of connecting an optical fiber to an optical waveguide with a large mode field diameter. Further, in connecting to an optical transmission device including each kind of a lens as well, when the mode field diameter of the optical fiber is expanded, there is an advantage that an alignment of optical axes is facilitated.
According to patent document 1, dopant with smaller core diameter is diffused by heating a fusion-spliced part again after fusion-splicing, when fusion-splicing the optical fiber. However, in this method, the connection loss can not be reduced in some cases. As reasons thereof, it can be considered that dopants of both optical fibers are diffused by local heating using electric discharge during fusion-splice, and deformation of a core itself is generated by a pushing operation of the optical fibers during fusion and a flow caused by surface tension of the fusion-spliced part; deformation is generated in the fusion-spliced part of the optical fibers in a fusion state, which is caused by an angle deviation and misalignment of optical axes in the fusion-spliced optical fibers; and it is difficult to selectively heat only the optical fiber side with a smaller core diameter.
In order to improve the aforementioned method, patent document 2 discloses a technique of achieving a fusion splice using a resistive heating heater, and performing TEC processing using a burner. A wider range can be heated by the resistive heating heater rather than by electric discharge heating, and therefore local dopant diffusion at a fusion-spliced part and core deformation can be suppressed. Further, TEC heating using burner is carried out in a temperature range of not more than a temperature (approximately 1300° C. or less) of allowing the deformation to be generated due to softening of the optical fibers, and not less than a temperature (approximately 500° C. or more) of allowing the dopant of a core part to be diffused to a clad part, requiring a time of about 3 minutes to 10 minutes.
According to the method of the patent document 2, there is a phenomenon that strength of the optical fibers is reduced by a long-period heating when applying TEC heating. Therefore, according to patent document 3, anneal heating is performed using a resistive heating heater or a burner, with a fusion-spliced part as a center.
The aforementioned patent documents 1, 2 describe the TEC heating after fusion splice. However, patent document 3 and patent document 4 describe a technique of applying TEC processing to a coating removed part of the optical fiber, to thereby obtain a connecting end face by cutting or polishing the coating removed part. According to patent document 4, opposed gas burners are used for the TEC heating, so that the TEC heating is performed by operating the burners in a longitudinal direction of the optical fiber in a state of pulling both sides of the coating removed part of the optical fiber. Since the optical fiber is hardly bent, thus allowing the heating to be performed at a high temperature, and requiring only several minutes-heating even in a case of a typical single mode fiber, with Ge as a dopant.
Note that shortening of the heating time is a common problem in a TEC technique, and patent document 5 provides a structure of eliminating scanning of the burners by lengthening a shape of a gas injection hole of each burner in a longitudinal direction of the optical fiber, thus allowing the processing to be shortened to 30 minutes which is half of a conventional required time, at a heating temperature of 1150° C. Patent document 6 provides a structure of arranging a plurality of gas injection holes in a row or arranging them two-dimensionally, so that a plurality of optical fibers can be simultaneously processed.
Patent document 1:
    Japanese Patent Laid Open Publication No. 1991-130705Patent document 2:    Japanese Patent Laid Open Publication No. 2003-75676Patent document 3:    Japanese Patent Laid Open Publication No. 2003-75677Patent document 4:    Japanese Patent Laid Open Publication No. 1992-260007Patent document 5:    Japanese Patent Laid Open Publication No. 2001-343549Patent document 6:    Japanese Patent Laid Open Publication No. 2004-157355
As described above, in the technique of the patent documents 1 and 2 wherein TEC heating is applied after fusion-splice, disturbance/deformation of a core and deformation of the optical fiber itself are generated in the fusion-spliced part, and it is impossible to selectively heat only the optical fiber of one side with smaller core diameter. Therefore, there is a limit in reducing connection loss. Further, this technique is a technique limited to a case of performing the fusion splice.
Meanwhile, the method of the patent documents 3, 4, wherein TEC heating is applied to the coating removed part of the optical fiber by a gas burner before fusion splicing, involves a problem that much heating time is required, although not limited to the fusion splice and having a general purpose of use. Although depending on the kind of the dopant of the optical fiber and an amount of the diffusion, about several minutes to several tens minutes is required. This is because there is an essential constraint such that a heating temperature can not be set to be so high as inducing softening the optical fiber, for suppressing the deformation of the optical fiber. Namely, even if the optical fiber is pulled and straightened during TEC heating, a heated part of the optical fiber is stretched and is made thin if the heated part is softened by heating at a high temperature. Further, since a gripped portion, etc., of both sides of the coating removed part of the optical fiber is gripped, refuse such as coating residue is stuck to the gripped portion, thus sometimes generating the angle deviation and misalignment of optical axes in the optical fibers of the gripped portion at two places. In this case, the optical fiber is deformed and bent at a heated part which is softened by heating at a high temperature, thus generating a large loss. Even if such a heated part which is deformed by heating, is formed into a connection end face by cutting the heated part, the connection loss can not be reduced, and in addition, there is also a problem that such a heated part can not be inserted into a connector ferrule. Accordingly, there is no choice but carry out TEC heating requiring a lot of time at a low temperature, thus making it difficult to reduce a cost, and meanwhile, there is a large connection loss in the end face obtained by cutting a portion with expanded core diameter by applying TEC processing to the optical fiber at a high temperature, and therefore practical use is difficult.